I have a '97 Yukon and the air conditioner is intermittent. It will start off blowing cold. After a 1/2 hour of driving it starts blowing warm, after that it cycles both ways without any kind of pattern.

I've had it to both a GM dealer and a local private shop. Both of those just let the truck idle out in the lot and it performed flawlessly. Then I took it to a third garage. They claim that the head control unit (the dash control module, where you turn on and control the heat & air) needs replacing and that its a very common problem on these trucks. Also they want to replace a clutch relay. In all about $375.

Has anyone replaced their dash control unit for this kind of problem? I'm just a little skeptical about their diagnostics. If this control unit is a big problem why didn't the other 2 places suggest it?

Welcome to the forum. It sounds like the third shop is just going through and telling you to start replacing parts until they find the root of the problem. I would be hesitant to let them do it unless they can prove that is what is wrong. Do you remember if the hot/cold cycling happens when you hit a bump? What I think is happening, based on your description, is that there is an intermittent short or a broken wire to your blend door actuator (the one that controls hot and cold). It is an easy fix if it were just the actuator. If it were the harness, it would be a PITA because the entire assembly has to come out to replace the harness. My recommendation would be to either take it back to the dealer or the earlier shop and let them relook at this. Or buy the Helm manual and go through it yourself.

You said at idle it performed flawlessly, right. Standing there, watching the A/C compressor (on 97's as you are facing the truck, it'll be right on top of the engine slightly off to the passenger side in plain site), the belt will be turning constantly (whether the A/c/Defrost is on or off). However in A/C/Defrost mode, it will engage the clutch and turn the compressor. You should see the end of the compressor start spinning when the clutch engages. It'll cycle for a little while, and then disconnect and repeat this process as needed. On hotter/more humid days it'll stay on longer (or constant).

Make a note of how it sounds when the clutch engages. When driving do you hear the same engagement/disengagement of the clutch? If not or intermittent, along car's line of thought, it could be a short or bad connection to the compressor.